In accordance with U.S. Pat. No. 2,920,971, the basic disclosure in the field of glass-ceramic production, such bodies are prepared via three general steps: first, a glass-forming batch is melted; second, that melt is simultaneously cooled and a glass body of a desired configuration shaped therefrom; and, third, that glass body is heat treated at temperatures above the annealing point and, frequently, above the softening point of the glass for a sufficient length of time to cause the glass to crystallize in situ. The heat treatment can be scheduled in such a manner as to control the size and, in some instances, the identity of the crystals developed. Therefore, in sum, the crystallization present in a glass-ceramic article can be the result of both the base composition of the precursor glass body and the heat treatment applied thereto.
Transparent glass-ceramic articles are well-known to the art; the classic study thereof being authored by G. H. Beall and D. A. Duke in "Transparent Glass-Ceramics," Journal of Materials Science, 4, pages 340-352 (1969). As is explained by those writers, glass-ceramic bodies will display transparency to the eye when the crystals present therein are considerably smaller than the wavelength of visible light or the birefringence within the crystals and refractive index difference between the crystals and the residual glass are very small. The authors noted three general composition areas in the aluminosilicate glass-ceramic system wherein highly transparent articles can be produced. In the first composition field, .beta.-quartz or .beta.-eucryptite solid solution comprises the predominant crystal phase. In the second region, spinel solid solution constitutes the primary crystal phase. In the third area, mullite makes up the principal crystal phase.
The production of transparent glass-ceramic articles containing .beta.-quartz or .beta.-eucryptite solid solution as the predominant crystal phase has been the subject of numerous patents and scientific papers. U.S. Pat. Nos. 3,241,985 and 3,252,811 are illustrative of such disclosures and the Beall and Duke paper cited above provides a definitive discussion of the composition and process parameters required to prepare such products, along with a study of the crystallization mechanism involved.
Glass compositions capable of being converted into transparent glass-ceramic articles containing zinc spinel, i.e., gahnite (ZnO.Al.sub.2 O.sub.3), as the predominant crystal phase are disclosed in U.S. Pat. No. 3,681,102. That patent also observed that the inclusion of a minor amount of Cr.sub.2 O.sub.3 in the compositions induced the development of a dark red fluorescence when the glass-ceramic bodies were exposed to ultraviolet and/or visible light. Thus, transparent glass-ceramic articles capable of demonstrating dark red fluorescence could be prepared from glasses consisting essentially, in weight percent on the oxide basis, of about 2-20% ZnO, 0-5% MgO, 8-25% Al.sub.2 O.sub.3, 0.01-1% Cr.sub.2 O.sub.3, 55-75% SiO.sub.2, and 2-12% ZrO.sub.2. The presence of MgO results in a zinc spinel solid solution (Zn, Mg) Al.sub.2 O.sub.4.
An extensive treatment of the production of glass-ceramic articles, including such articles exhibiting transparency, having base compositions in the Al.sub.2 O.sub.3 -SiO.sub.2 system is provided by J. F. MacDowell and G. H. Beall in "Immiscibility and Crystallization in Al.sub.2 O.sub.3 -SiO.sub.2 Glasses," Journal of the Ceramic Society, 52(1), pages 17-25 (1969). In that paper the authors elucidate the phase separation and crystallization mechanisms involved in the conversion into glass-ceramic bodies of both binary Al.sub.2 O.sub.3 -SiO.sub.2 glasses and ternary glasses, i.e., Al.sub.2 O.sub.3 -SiO.sub.2 glass compositions to which modest amounts of modifying oxides are added, specifically noting BaO, CaO, and Na.sub.2 O as operable modifying oxides. The writers observed that the binary glasses readily phase separated such that fast quenching of the glass melts was demanded to obtain crystal-free precursor glass bodies. The addition of the modifying oxides tended to inhibit phase separation during cooling of the melt, thereby rendering it easier to secure homogeneous glass bodies. Nevertheless, the quantity of modifying oxide added must be carefully controlled. Hence, the molar ratio modifying oxide:Al.sub.2 O.sub.3 must be maintained less than 1 or phase separation will not occur with the consequence that a fine-grained glass-ceramic body cannot be formed.
In contrast, the writers noted that small additions of such glass network formers as B.sub.2 O.sub.3, BeO, GeO.sub.2, TiO.sub.2, and ZnO to the ternary system of compositions promoted ready separation of the glass into two phases as the melt cooled, thereby rendering the glass easily crystallizable in situ with the resultant glass-ceramic body having mullite or a mullite-cristobalite assemblage as the predominant crystal phase. The authors explained that the cations of the network formers are capable of occupying positions of fourfold oxygen coordination in place of Si.sup.+4 or of sixfold coordination in a three-dimensional glass network. Because of that capability, the writers posited two possible explanations for the positive effects which the network former cations exerted upon phase separation and crystallization: (a) tetrahedral Si.sup.+4 sites were occupied by the network former cations, thereby forcing some Al.sup.+3 into octahedral coordination; or (b) the network former cations directly became a part of the immiscible, octahedrally-based aluminous (mullite-forming) network. Nucleating agents may be included in the compositions. For example, that Nb.sub.2 O.sub.5, SnO.sub.2, Ta.sub.2 O.sub.5, TiO.sub.2, WO.sub.3, and ZrO.sub.2 are commonly in sixfold coordination with oxygen. Those ingredients can contribute along with Al.sub.2 O.sub.3 to the formation of an immiscible octahedral glassy component upon cooling the glass melt, which ultimately results in internal nucleation. Cr.sub.2 O.sub.3, however, is nowhere mentioned in the paper.
Unfortunately, the conventional binary and ternary aluminosilicate glass compositions operable as precursors for the preparation of glass-ceramic bodies containing mullite as the predominant crystal phase are characterized by high melting and working temperatures, customarily at least 1800.degree. C. MacDowell and Beall in the above paper employed melting temperatures of 1850.degree.-1900.degree. C. Not only do such elevated temperatures impose very high energy costs, but also attack upon the refractory materials of the melting unit becomes very serious.
Therefore, the primary objective of the instant invention is to provide glass-ceramic articles containing mullite as the predominant and, preferably, sole crystal phase which are substantially and, desirably, totally transparent, which can be prepared from parent glass compositions capable of being melted at temperatures no higher than 1650.degree. C., and which can be crystallized in situ at high glass viscosities, viz., about 10.sup.9 -10.sup.12 poises.
The use of dopants in glasses and glass-ceramics to impart color, luminescence, fluorescence, or other physical phenomena thereto is well-recognized in the art. Such dopants have included chromium, cobalt, copper, erbium, iron, manganese, nickel, praeseodymium, terbium, tin, uranium, vanadium, and tungsten. Accordingly, an objective complementary to the above-described primary objective of the invention would be to provide such mullite-containing, glass-ceramic articles which are doped with ingredients to confer color and/or luminescence and/or fluorescence thereto.
It has been observed that the Cr.sup.+3 ion, when placed in an appropriate octahedral ligand field, will exhibit fluorescence in the infrared region of the radiation spectrum. Hence, that phenomenon is the foundation of the alexandrite (BeAl.sub.2 O.sub.4) single crystal laser recently described in the literature ("Alexandrite Lasers: Physics and Performance", J. C. Walling, Laser Focus, February, 1982). The paper also described the role of Cr.sup.+3 excited state transitions in fabricating a tunable laser device based upon Cr.sup.+3 -doped alexandrite crystals. That disclosure has prompted the suggestion that a tunable infrared laser could be designed utilizing Cr.sup.+3 -doped glasses or glass-ceramics. High optical quality, i.e., a very low level of haze, would be of paramount importance.
Another proposed application for Cr.sup.+3 -doped glasses or glass-ceramics would involve their use in the fabrication of luminescent solar collectors for use in conjunction with silicon photovoltaic cells. The concept contemplates employing such collectors to convert broad spectrum sunlight to the near infrared portion of the spectrum and then guide that radiation to silicon photovoltaic cells positioned around the periphery of the collectors. Such a design would significantly reduce the quantity of silicon needed to produce a given amount of electricity, since silicon photovoltaic cells operate most efficiently in the near infrared regime of the spectrum. For that application, easy formability, chemical durability, broad temperature range, and relatively low cost, as well as excellent optical quality, would be premium qualities in addition to good fluorescence.
To investigate the possible utility of Cr.sup.+3 -doped, transparent glass-ceramic bodies in such applications as tunable lasers and solar collectors for use with silicon photovoltaic cells, compositions within each of the above-discussed three systems were doped with various levels of Cr.sub.2 O.sub.3. Subsequent testing of samples indicated that the mullite-containing glass-ceramics were significantly more effective in converting ultraviolet and visible radiation to the infrared wavelengths where silicon photovoltaic cells operate most efficiently. Upon crystallization to mullite-containing glass-ceramics, the precursor glass body changes in color from a deep green to a transparent gray-brown, this phenomenon suggesting that the Cr.sup.+3 ions undergo a change in coordination during that operation.
The mullite-containing specimens demonstrated a further advantage over the spinel-containing bodies. Thus, in contrast to the spinel-containing samples, the mullite-containing specimens displayed broad absorption in the visible region of the radiation spectrum, as evidenced by the gray-brown color. Spectral measurements suggest that there is relatively little overlap between the absorption and fluorescence wavelengths, this feature being of special significance in a material being considered in an application such as a luminescent solar collector.
Therefore, a specific objective of the instant invention is to provide substantially and, preferably, totally transparent glass-ceramic articles containing mullite as the predominant and, most desirably, sole crystal phase from parent glass compositions which can be melted at temperatures no higher than 1650.degree. C., which can be crystallized in situ at high glass viscosities, viz., about 10.sup.9 -10.sup.12 poises, and which, when doped with Cr.sup.+3 ions, will yield glass-ceramic bodies exhibiting broad absorption over the visible region of the radiation spectrum, strong fluorescence in the red and near infrared regions of the radiation spectrum upon being exposed to ultraviolet and/or visible radiation, and relatively little overlap between the absorption and fluorescent wavelengths.